"Shadowhunters" already has a pretty weird thing in common with "Game of Thrones," given that they both spent a good chunk of their first season letting a brother and sister hook up like it was no big deal. Now, with the addition of Will Tudor as Sebastien, the two shows have something way less creepy in common.

Tudor spent a brief stint on "Game of Thrones" as Olyvar, Loras' (Finn Jones) squire/lover, who outed him to the religious radicals of King's Landing. Now, he'll be taking on a distinctly more villainous role on "Shadowhunters," as a character we've been dying to meet since the show began: Sebastien.

The official casting announcement from Freeform describes Sebastien as "a charming, brilliant and highly educated Shadowhunter with unrivaled fighting abilities. Originally from a foreign Institute, he finds his way to the New York Institute after befriending one of the show’s main characters."

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We've got some theories about just who he'll be befriending, based on what we know from the books -- but they're incredibly spoiler-ridden, so we'll resist speculating too much.

Needless to say, Sebastien is a game-changer, who will shake things up at the Institute for better and for worse. He'll also be changing the dynamics of several relationships along the way -- so be on the lookout for clues about Sebastien's true identity and agenda when he makes his way onto your screen.

"Shadowhunters" airs Mondays at 8 p.m. ET/PT on Freeform.

Posted by:Lindsay MacDonald

Lindsay MacDonald is a Los Angeles based entertainment reporter with an affinity for CW superheroes. She graduated from Pepperdine University with a major in Media Studies and a borderline unhealthy obsession with TV in 2012. She would much rather spend the day binge-watching ‘The Flash’ or sorting ‘Game of Thrones’ characters into Hogwarts houses than venturing outdoors. TV words to live by: “Never ignore coincidence. Unless, of course, you’re busy. In which case, always ignore coincidence.”